do i need a certain weight or type of solder for wires to be soldered together in my engine bay, or will 60/40 rosin core be fine?

 

that will be fine.... all the rosin does is make the solder flow better.... once that is all burnt off then its just purely solder

 

63/37 is preferred. It is known as the Eutectic ratio. It means it has less time in "Plastic" state which means less chance of a cold joint. Put simply, 63/37 makes better joints.

If you are having problems getting everything up to temperature (eg. larger wires), then you might look at getting some "Eutectic Silver" solder. For some reason unbeknownst to me, silver solder has a lower melting temperature then Tin, Lead, or Silver. Mixing the three together in the eutectic ratio can produce a solder that melts at a much lower temperature which is perfect for working outside, with wind cooling the iron, and the solder snaps from liquid to solid and back again instantaniously.